In general, for the purpose of carrying out cutting, soldering and welding of workpieces by means of high-power laser beam, high-power laser beam emitted from laser oscillator is injected into optical fiber 2 through input-side lens unit 1 to be transmitted to working side as shown in FIG. 1, and said laser beam is irradiated, through said optical fiber 2, from output-side lens unit 3 onto work 4 thereby to carry out a predetermined working thereon. The construction is such that said high-power laser beam from said input-side lens unit 1 is focused on the end surface of said optical fiber 2, while reflected laser beam from said work 4 is, through said output-side lens unit 3, focused onto the other end surface of said optical fiber 2.
There has been a problem however that though high-power laser beam is to focus on the end surface of optical fiber 2, it may focus on other portion than said end surface, for example on adhesives, which adhesives may be heated by high-power laser beam and burned out.
For eliminating such difficulties, a proposal concerning the end portion of optical fiber has been proposed for example by Japanese Utility Model Publication No.91-17284.
More specifically, as shown in FIG. 2 showing the prior art end portion of optical fiber, the end portion of optical fiber 2 formed by central core and outer clad is covered on its outer side by a spacer sleeve 5 of heat resisting material, which spacer sleeve 5 is further covered by a heat-resisting protective sleeve 6. End surface 5a of said spacer sleeve 5 is located inside at a predetermined distance from the end surfaces of optical fiber 2 and said protective sleeve 6, thereby to form a space where there is no substances apt to be burned by high-power laser beam, in order to prevent any burning damage due to laser beam even when it focuses at any place other than said end surface of optical fiber 2.
However, since said heat-resisting spacer sleeve 5 is formed from transparent body or translucent body, the portion of high-power laser beam which is not filled in optical fiber 2 caused by erroneous injection thereinto or by reflected laser beam from the surface of work 4 penetrates into said spacer sleeve 5, with disastrous results of burning of said protective sleeve 6 and even optical fiber 2 itself. Further, since the end surface 5a of said spacer sleeve 5 is a crosscutting surface which is perpendicular to axial direction of optical fiber 2, i.e. substantially to the direction of high-power laser beam, high-powerlaser beam portion which was diverted from the end surface of optical fiber 2 and which reached said end surface 5a gains a higher level of energy density thereby to burn out said spacer sleeve 5 itself.